1. Field of the Invention
The present invention relates to modular concrete culverts and, more particularly, the manufacture of a three-sided concrete culvert with prestressing tendons running therethrough.
2. History of the Prior Art
The prior art is replete with culvert designs affording a passage way for water or traffic beneath embankments, roadways and the like. The technology of culvert construction extends back into technological antiquity. Culverts were originally constructed for free water flow and built from available stones and rock indigenous to the area of construction. The last century has obviously seen marked advances in culvert construction and design. In contemporary culvert construction there are two major approaches, to wit: construction in place and installation of prefabricated sections. There are, of course, hybrid assemblies and many of these approaches have been found patentable by the U.S. Patent and Trademark Office as described in various references set forth below.
Culvert construction like any other advanced construction technique requires a solid foundation. Such foundations come in many forms including slabs, piers, footers and/or beams. The type of foundation obviously depends on the area of use of the culvert, the necessary flow area therethrough and the loading characteristics therefor. Small drainage culverts are, for example, generally formed of metal or concrete pipes sometimes placed directly on earthen beds, preferably with a gravel or granular base. Larger culvert installations for accommodating massive amounts of water flow generally utilize a concrete or steel culvert body of either a precast modular or cast in place construction that is disposed upon a wide slab, multiple piers or beams. Both slabs and beams have been used for decades with steel and concrete culverts of both precast and cast in place varieties. Moreover, the interconnection between the culvert body and the foundation or base has itself been the subject of numerous developmental activities including several prior art patents. For example, U.S. Pat. No. 494,576 to Lewis shows an arcuate culvert body of the prefabricated variety disposed upon a base comprised of two beams and/or footers in support thereof. This very old reference clearly teaches the utility of resting a metallic culvert body on such a foundation constructed along a stream. The arcuate construction was once a very popular design due to the fact that the loads which could be supported were evenly distributed around the arcuate culvert in accordance with basic principles of structural engineering. Such an arcuate design did, however, require side support of the culvert section by both the earthen fill regions laterally thereof and the culvert foundation. In many instances, the culvert foundation or footers included grooves in which the sidewalls of the culvert were inserted as shown in the Lewis patent. This type of culvert construction was common for many decades until the advent of more advanced structural technology.
When the structural limitations and advantages of concrete were more fully known and the portability of concrete via portable mixing units and the like was established, much larger and stronger culvert sections could be provided. Again, two types of concrete culverts found commercial success, to wit: the box culvert and the three-sided culvert. The box culvert, as the name defines, is comprised of four generally orthogonal concrete walls defining an internal passage therethrough. Box culverts could easily be poured at the site of the culvert construction by first pouring a slab foundation and side and top walls thereover. Technological advances provided such concrete structures and more lightweight configurations which could then be cast in modular form at locations remote from the ultimate site of installation. These modular box culverts then became a very popular article of commerce in conjunction with the more conventional pipe culvert section which also found widespread popularity. The box culvert, however, provided one feature which a circular pipe culvert did not, that was a substantially uniform cross-sectional area with vertical rise in water level. Further advances resulted in the utilization of the three-sided culvert module which could be manufactured and transported to the installation site. The culvert foundation, either of the slab or beam variety was necessary and had to be formed, reinforced and poured separately from the upper culvert body portion anyway. Since time at the installation site is obviously more expensive than time in a manufacturing facility, the three-sided modular culvert comprising a top slab and two generally parallel vertical wall portions became very popular. One such culvert is set forth and shown in the Humes Manufacturing Catalog. No. CPR14-70R dated 1970. Such three-sided culverts having parallel side walls and a planar top surface, found widespread popularity and have been installed for years on a variety of culvert foundations for a number of culvert installations and applications.
There have also been several designs deemed patentable by the U.S. Patent and Trademark Office in view of the prior art references that were then before the Office in the area of three-sided culverts. For example, U.S. Pat. No. 4,564,313 was issued to Nisswander in 1986 for a three-sided, rectilinear bridge or culvert structure comprising a pair of vertical sidewalls set in preform or cast-in-place footers. Although that patent does not refer to or discuss the three-sided culvert installation such as that shown in the Humes catalog referred to above, it does address various concrete and corrugated metal covered culverts, bridges and arch bridges existing in the prior art. As stated therein, reinforcing bar, or "rebar", was commonly used in the concrete and numerous prior art references are cited.
One aspect of a rectilinear, three-sided culvert is the haunch, preferably disposed in each internal corner to assist distribution of load. Haunches too are well known in the prior art as taught in the Humes catalog. Likewise, the Lockwood U.S. Pat. No. 4,595,314 also issued in 1986 for a culvert section including vertical parallel spaced concrete sidewalls integrally connected by an arcuate concrete top wall having a curved inner surface. Again, the structural aspect of the culvert in the modular configuration is shown to include the primary utilization of rebar. The FitzSimmons U.S. Pat. No. 4,537,529 issued in 1985 also shows an arch shaped, precast, reinforced concrete element again using a reinforced steel concrete body. The FitzSimmons U.S. Pat. No. 4,558,969 is a 1985 patent teaching a hinge for use with large, precast, overfill load supporting structures. The precast load support structure is again taught to be constructed with reinforced steel and in an arcuate configuration for transferring various lateral and vertical loading. Specific examples are, in fact, given in this and other references relative to the size of reinforcing bars as well as the center-to-center spacing thereof. These are examples of the state of the prior art relative to precast, modular, culvert configurations. It may be noted that the utilization of rebar though effective has certain limitations and structural implications. However, the use of rebar is not limited simply to precast modular elements.
The more conventional prior art of culvert design is also replete with bridge and culvert forms which allow the formation of a concrete culvert structure at the installation site. For example, the Jennings U.S. Pat. No. 3,693,927 is a 1972 reference showing a release plate for a collapsible culvert form. This particular construction box culvert is formed by first pouring a concrete base having footings integrally constructed on the floor or ground surface on which a roller unit form may be supported. Consistent with this invention, concrete can be poured in the movable forms once the base has been set and this particular reference illustrates again the utilization of the footer design having a groove formed therein for this cast-in-place configuration. Such cast culverts and the utilization of forms does, however, date back many decades prior to the Jennings patent. For example, the Schroeder, Jr. U.S. Pat. No. 2,041,267 teaches the utilization of a series of forms for a culvert assemblage.
Another prior art patent of earlier design in the culvert form area is set forth and shown in the Scott U.S. Pat. No. 2,265,871 showing a bridge and culvert form. The forms are shown to be collapsible and removable for framing relative to pouring at the installation site.
Such culvert designs have thus addressed specific configurations of cast, precast, metal, boxed, U-shaped and arcuate configurations. In all of the concrete structures set forth and shown in the prior art, the utilization of rebar is the primary means of structural support. It is known in the prior art, however, to use prestress and post-tensioning tendons in concrete structures. Such prestressing and tensioning tendon technology is well established, particularly in the construction industry. The tendons are disposed within the forms either in a taut configuration where the cables themselves are exposed to the concrete poured therearound for bonding thereto or the tendons are sheathed for protection from the concrete so that post-tensioning can occur. There are numerous structural and functional differences, but there is one similarity. Most conventional prestressing and post-tensioning technology pertains either to cast-in-place beam and slab concrete structures or concrete pipe production techniques. The utilization of prestressed or post-tensioned tendons in the fabrication of modular culverts has not been addressed. For example, the Kinney U.S. Pat. No. 3,005,469 teaches prestressing concrete pipe with prestressing tendons which are wrapped around the outside of a pipe, tensioned the desired amount and secured in place for the purpose of maintaining the pipe in compression. The Prosser U.S. Pat. No. 2,164,625 teaches a concrete pipe and method of producing same again for providing a relatively thin wall but very strong assemblage. The general object of such structures is to provide a concrete element and method of production by which the strength of the reinforcement therein may be utilized to prevent rupture during use in a manner that conventional rebar reinforcement is not satisfactory. Moreover, such fabrication techniques can be provided in a cost-saving and weight saving configuration. The Miller - Osweiler U.S. Pat. No. 2,236,107 likewise teaches a reinforced pipe and method of making same which is a 1941 reference utilizing such tensioning techniques.
It would be an advantage therefore, to overcome the problems of the prior art modular culvert assemblies by providing a method and apparatus for constructing modular concrete culverts in lightweight configuration capable of withstanding greater loads and for less cost. Such a structure is provided by the present invention which teaches the utilization of a three-sided culvert, the top of which is fabricated with prestressed tendons disposed therethrough. The tendons enable the structure to be constructed for withstanding loads much greater than that ordinarily affordable with conventional rebar reinforced slabs and at a weight substantially less than the weight of a modular structure adapted for equivalent loading characteristics. This reduces the cost of manufacture, transportation costs and installation expenditures. One foundation for the culvert is also provided in a configuration which maximizes flow area while reducing costs. Moreover, with the present invention a plurality of modular culvert sections can be fabricated simultaneously utilizing the innovative teachings and techniques of the present invention as further described below.